The term foreign bodies comprises all solid materials that are undesired in products, especially food products, originating from the product or not, such as bone fragments, bits of glass, rubber, gravel/stone, hair, insects, etc.
In the international application published as WO01/20311, an apparatus for detection of foreign bodies in materials is disclosed which is fast, non-invasive and non-destructive. Electromagnetic signals having at least two different frequencies pass through the material and the transmitted and received signals are analyzed and compared to a predetermined reference value, obtained from a reference sample, for each frequency. A presence of a foreign object in the material will create a difference in damping and/or phase shift between the material and the reference sample.
A drawback of the prior art is that the described method only measures the dielectric property ∈, which means that it is difficult to detect a foreign object in a material if the dielectric contrast between the object and the material is small.
Further drawbacks with the described method is that the resolution of the method is limited by the microwave wavelength, which in some cases results in a too small resolution, and increasing frequency reduces the wavelength and therefore increases the resolution at the cost of a limited penetration depth. The antenna patterns used in the apparatus are sensitive to adjacent products, which leads to crosstalk. Furthermore, measurement results are inherently temperature dependent.
Furthermore, the U.S. Pat. No. 3,910,124 A discloses a system including two different energy sources, viz. at least one microwave transmitter and at least one ultrasonic transmitter. The energies emitted from these different sources are said to complement each other in that the ultrasonic energy is sensitive to laminar conditions while microwave energy is sensitive to density changes, in the search for intra part or subsurface anomalies. Also, it is stated that ultrasonic energy penetrates a metal component with greater facility, while microwave energy will penetrate any non-metal component.
However, still, the mere complementary use of microwave and ultrasonic energies will not increase the resolution while maintaining a sufficient penetration depth in order to achieve the desired detection of a change in material composition, such as foreign bodies in food products.